Magmatic to hydrothermal REE mineralization in Mesoproterozoic ankerite carbonatite, Swartbooisdrift, NW Namibia

Strong local REE enrichment is observed in Mesoproterozoic ankerite carbonatite exposed at Swartbooisdrif, NW Namibia (Drüppel et al., 2005). The carbonatite dykes transect older anorthosites of the Kunene Complex and are themselves surrounded by sodalite-rich Na-metasomatic aureoles. The magmatic assemblage of ankerite, magnetite, pyrochlore, burbankite, and apatite indicates that the carbonatite magma was rich in Fe, Nb, P, Na, Sr, and LREE. Primary REE hosts are euhedral burbankite, pyrochlore, and apatite, which can be enriched in irregular layers formed during flow-banding.

Along cracks burbankite is altered to secondary burbankite and carbocernaite. With progressive alteration, burbankite and carbocernaite were partially to completely replaced by complex intergrowths of fluorcarbonates (mainly bastnaesite, hydroxylbastnaesite, parisite and synchysite), ancylite, monazite, barite, strontianite, celestine, calcite, fluorapatite, and/or hematite. Similar mineral assemblages also occur as discrete REE-rich pods and schlieren, suggesting fluid-related remobilization of REE and P. These zones are mainly restricted to the ankerite carbonatite, indicating locally restricted REE mobility.

The composition of early metasomatic and later hydrothermal fluids was determined by microthermometry and synchrotron-micro-XRF analysis of primary fluid inclusions in sodalite and secondary fluid inclusions in ankerite. Early high-temperature (> 550°C) sodalite-forming fluids were highly saline alkaline brines (19-30 wt% NaCl eq.) containing minor Sr, Ba, LREE, Nb, S, K, and Fe. Secondary fluid inclusions in ankerite, entrapped at lower temperatures of c. 400-500°C, are NaCl-poor (4-6 wt% NaCl eq.) and strongly enriched in Sr and REE. These fluids are presumably related to the late REE remobilization.

Drüppel et al. 2005, J. Petrol., 46, 377–406.